The present invention relates to a device by which the volume of respiratory gas in an anaesthesia circuit and changes in such gas volume can be accurately, rapidly, and easily determined. The invention is particularly suited for use in an anaesthesia circuit operating in the closed circuit mode to enable an appropriate control apparatus or an attending anesthesiologist or other medical personnel to establish and maintain the gas volume in the circuit to a desired, generally constant, level.
Apparatus for administering an inhaled anaesthetic agent to a patient typically includes an anaesthesia machine in which the anaesthetic agent is entrained in a flow of carrier gas. The carrier gas is usually a mixture of oxygen and nitrous oxide or air. The carrier gas and entrained anaesthetic agent from the anaesthesia machine are provided to an anaesthesia circuit for delivery to a patient in the course of his/her respiration. The anaesthesia circuit is typically operated in one of two modes: a semi-closed mode or a closed circuit mode. In the semi-closed mode, the amount of gas supplied to the circuit from the anaesthesia machine exceeds that taken up by the patient. The excess gas is discharged from the circuit to a scavenging system through a pressure limiting, or "pop off", valve during the exhalation phase of the patient's breathing.
In the closed circuit mode the respiratory gases in the circuit, including gas exhaled by the patient, are not discharged from the circuit but are retained in the closed circuit for recirculation to and from the patient. To absorb the carbon dioxide contained in the respiratory gases exhaled by the patient, a CO.sub.2 absorber employing a suitable absorbent, such as soda lime, is provided in the circuit.
In a closed circuit anaesthesia system, once the patient is brought to near equilibrium with the respiratory gas contained in the anaesthesia circuit, the amount of gas supplied to the circuit is only the small amount necessary to provide the patient minute oxygen consumption plus some minimal level of anaesthetic agent.
Closed circuit anaesthesia systems have numerous advantages over semi-closed or open systems. These include decreased use of carrier gas and anaesthetic agent. This results in decreased costs from the lessened consumption of the carrier gas and the anaesthesia agent. The amount of environmental pollution is lessened because of the closed nature of the circuit. In a closed circuit system, the patient minute oxygen consumption can be rather precisely determined since, with the circuit at equilibrium, the amount of oxygen supplied to the closed circuit in the gas flow from the anaesthesia machine will be the same as that which the patient is consuming.
However, closed circuit anaesthesia circuits have not achieved as wide spread usage as their advantages would warrant. This is due, in considerable measure, to the fact that a closed anaesthesia circuit requires continuous monitoring of the volume and composition of the respiratory gas in the circuit and careful maintenance of those characteristics by controlling the amount and composition of the gas flow supplied to the closed circuit.
The gas volume in a closed circuit anaesthesia circuit is evident from a bellows assembly of the attached ventilator. The bellows assembly includes an expandable, pleated bellows, formed of rubber or other flexible material. The bellows is connected in the closed circuit for receiving and discharging gas respired by the patient. The bellows is contained in a surrounding rigid housing, typically formed of clear plastic. To cause or assist the patient to inhale respiratory gas, a separate, driving gas is supplied to the housing to compress the bellows and deliver respiratory gas in the closed circuit to the patient. As the patient exhales, the driving gas is allowed to exit the housing so that the bellows may expand to receive respiratory gas exhaled by the patient. The process is then repeated for the next breathing cycle of the patient. The bellows is typically compressed downwardly during inhalation and expands upwardly during exhalation, and the housing surrounding the bellows is cylindrical in nature, with a vertical volumetric scale.
The anesthesiologist, or others, using an anaesthesia system in a closed circuit mode, adjusts the gas volume in a closed circuit system so that the bellows rises, with the patient's exhalation, to an intermediate position on the vertical scale of the housing. Changes in the gas volume in the closed circuit are determined by observing changes in the position of the bellows in the housing at the end of successive breathing cycles of the patient. For instance, if the bellows is gradually rising higher and higher with each exhalation of the patient, the volume of gas in the closed circuit is increasing. The anesthesiologist must slightly decrease the gas supply of one or more of the gases supplied to the closed circuit to restore and maintain the desired constant volume in the circuit. If, on the other hand, the bellows is gradually returning to a lower and lower position in the housing with each exhalation, the anesthesiologist will need to add gas from one or more of the gas supplies to the closed circuit. When the desired goal of maintaining the volume of gas in the closed circuit at a constant level is being achieved, the bellows will return with each exhalation to the same intermediate position in the housing.
The desired composition for the respiratory gases in the closed circuit is determined by measuring the oxygen concentration in the closed circuit, typically the inspired oxygen concentration (FiO.sub.2), and the amount and composition of the gas supplied to the circuit is adjusted to maintain the oxygen concentration at a desired level.
The need for continuous monitoring of gas volume and composition and careful maintenance of these characteristics places a corresponding burden on the anesthesiologist attending a patient to observe the magnitude of the gas volume, the nature of changes therein, and composition of the respiratory gas in the closed anaesthesia circuit and to properly operate the appropriate gas supply values in response to such observations. These requirements are in addition to other demands on the anesthesiologist, for example, to monitor the physiological condition of the patient during a surgical or other medical procedure.
The foregoing burden has caused anesthesiologists to employ other modes of operation instead of closed circuit anaesthesia circuits, in spite of the advantages of the latter.